1. Field of the Invention
The present invention relates to data transmission methods and systems, and devices used therefor and, more specifically, to a method, system, and device for transmitting isochronous data such as digitalized audio and video by using a radio signal as a transmission medium. Such transmission takes place in a media access control for LAN, or in a data link layer and network layer.
2. Description of the Background Art
Conventionally, in attempt of realizing a network enabling transmission of isochronous data such as digitalized audio and video by using a radio signal as a transmission medium, a technique of transmitting data over a millimetric radio wave ATM-LAN has been disclosed in Shingakugihou (IEICE Technical Report) RCS97-207, January, 1998. Described briefly in the following is the millimetric radio wave ATM-LAN.
FIG. 11 is a diagram structurally showing the conventional millimetric radio wave ATM-LAN. In FIG. 11, APs (AP1 to AP3) each denote an access point and serve as a master or central station. STAs each denote a terminal, and is controlled by any one AP. Communication between any STA and another STA is always carried out via the AP. The AP is connected to a wire-link network such as ATM. Therefore, the AP, in some cases, communicates with any STA controlled by other AP also connected to the wire-link device or network.
The AP controlls every STA in the area communicable by using millimetric waves (e.g., 60 GHz band) as the medium for radio transmission, and also manages radio transmission resources. Specifically, the AP inquires about an access request from any STA under control, and allocates, to the STA, the resource for radio transmission. For such allocation, the AP gives higher priority on isochronous data such as audio and video so that the transmission of such data completes within a predetermined time period with allowance. On the other hand, to the asynchronous data, allowed to be transmitted with some delay, the AP allocates the radio transmission resources so as to allocate the time that will remain after the isochronous data transmission is complete.
The AP then transmits a polling, in a predetermined cycle calculated by itself, to the STA allocated with the radio transmission resource for isochronous data, and permits data transmission therefrom. Upon permission, the STA that has received the polling from the AP transmits a data frame, and the AP receives the data frame. If a network address of the received data frame is addressed to other STA under control, the AP transmits the data frame thereto. If the network address is addressed to any STA in the wire-link network, the AP transmits the data frame thereto.
FIG. 12 is a diagram showing a sequence when an arbitrary STA in FIG. 11 transmits isochronous data to other STA via the AP. Hereinafter, the STA of the transmitting end and the STA of the receiving end are referred to as a transmitting STA and a receiving STA, respectively. In FIG. 12, the upper part shows a sequence for normal transmission and the lower part shows a sequence for transmission failed in its first transmission.
Referring to the upper part of FIG. 12, the AP first transmits a polling to the transmitting STA. In response thereto, the transmitting STA transmits data. The AP receives and transmits the data to the receiving STA.
Referring to the lower part of FIG. 12, the AP transmits a polling to the transmitting STA, waits for a predetermined time passes, and if not receiving data from the transmitting STA, transmits another polling to the transmitting STA and waits for data transmission therefrom.
Using the millimetric radio wave ATM-LAN described above, a full-duplex operation path must be formed in order to enable simultaneous use of an ascending transmission path directed from STA to AP and a descending transmission path directed from AP to STA. Herein, the full-duplex operation path is formed frequency-division-multiplexing using two radio channels.
Such bidirectional communication (full-duplex operation) through the frequency-division-multiplexing is easy to be implemented in a band having wide transmission range such as millimetric wave band. In the microwave band, however, usually only one channel can be used because a transmission speed in each channel reaches some megabits per second. Therefore, in the microwave band, one channel is generally multiplexed in a time-sharing manner so as to achieve bidirectional communication (half-duplex operation). In such case, a preamble bit, or the like, needs to be added at the head of an ascending burst and a descending burst for synchronization in radio transmitting/receiving devices. The preamble bit, in many cases, exceeds 100 bits. The overhead caused by the preamble bit is thus increased to an amount that can not be ignored with respect to data amount of control information such as polling. Consequently, the response time in the half-duplex operation disadvantageously reaches a level several times longer than that presumed in the full-duplex operation.
Further, in comparison of transmission quality with wire transmission, radio transmission shows increment in a bit error rate by more than two digits, which should be taken into account. Different from the bit error rate, a frame error rate, i.e., a rate of occurrence of error somewhere in a data frame, much depends on the number of bytes of the data frame. The more the number of bytes of the data frame is, the worse the frame error rate becomes.
The maximum frame lengths defined by the IEEE 802.3 standard and the IEEE 802.5 standard for the wire-link LAN are 1,500 bytes and 4,429 bytes, respectively. In comparison, the maximum frame length for the radio transmission is generally around 256 bytes. In this case, overhead caused by the data relevant to control information such as polling, in addition to that caused by the preamble bit, can not be ignored.
A relay device may be placed in between AP and STA in such case that the radio wave can not be directly transmitted due to any obstruction or too long distance therebetween. Even in this case, if frequency that can be used for radio transmission is limited, the communication between the AP and the relay device, and between the relay device and the STA are all done in the same radio transmission frequency. Accordingly, ascending and descending data in each section of transmission path should be multiplexed in a time-sharing manner to avoid data collision during transmission. Herein, by adopting the medium access control of a centralized-control-type using the polling, transmission control is easily done in a time-sharing manner. In such case, however, the transmission band is further reduced by half of the half-duplex operation. The overhead caused by the preamble bit and data relevant to the control information such as polling thus becomes more serious.